A manufactory typically comprises many work centers and workers. Each work center is equipped to perform a certain function. For example, at one work center drilling may be performed, and at another work center cutting may be performed. Furthermore, each worker in the manufactory may be qualified to perform only certain types of functions. For example, one worker may be qualified only to paint, and another worker may be qualified both to paint and to drill.
Generally, a manufactory can produce a variety of finished products in response to customer orders. The manufacturing of each finished product typically is divided into many tasks. Tasks, and the order in which they must be performed, may vary from product to product. The tasks of various finished products can be in production simultaneously. This type of manufactory is called a "job shop."
The problems associated with the scheduling of tasks within a job shop have long been recognized in the art. Many managers are unable to develop workable schedules because they have no means of accurately determining the "queue time." The queue time is the time a task waits because resources (e.g., workers and work centers) are occupied with other tasks. Without accurate schedule information, a manager has no means of assessing and making realistic due date commitments for prospective customer orders. Consequently, workers frequently are reassigned or asked to work overtime to compensate for unanticipated bottlenecks. These disruptions are costly.
Existing scheduling techniques attempt to overcome these disruptions by using planned queue times, which are a manager's estimates of queue times, and time buckets, which are fixed periods of time (typically a week) into which the planning horizon is divided. Based on the planned queue times, each task is assigned to a time bucket. If all tasks assigned to a time bucket cannot be completed during that time bucket, certain systems (called "finite loading") reassign tasks to other time buckets; other systems (e.g., MRP and MRP II) do not attempt to make these changes.
These approaches to scheduling result in infeasible and unrealistic schedules for several reasons. First, the planned queue times are typically very inaccurate. Second, the time buckets must be at least as long as the time to do the longest task and in most cases they are much longer. Third, only one of the two important resources, workers and work centers, is considered in the schedule. Fourth, since either workers or work centers are assigned to tasks before the schedule is developed, unanticipated bottlenecks frequently arise on over-committed resources. Fifth, since revenues and costs are not explicitly considered, the schedule objectives are unrealistic. Finally, since the computer time necessary to develop a schedule is typically long, revisions are infrequent and the schedule does not reflect current production conditions.
Accordingly, there is a need for a scheduling system that develops accurate, feasible, minimum cost schedules in a timely manner that allows revisions to reflect current production conditions.